Delayed hypersensitivity in vivo is associated with macrophage proliferation which occurs concomitant to the development of lymphocyte sensitization. We have observed that antigen-stimulated guinea pig lymph node lymphocytes produce a soluble factor which induces the proliferation of nonimmune peritoneal and alveolar macrophages incubated as monolayers; partial purification of this factor using Sephadex chromatography indicates an approximate molecular weight similar to that of migration inhibitory factor (MIF); however, vacuum dialysis prior to chromatography removes MIF and, therefore, distinguishes between the two factors. Experiments are proposed to further purify this mitogenic factor and to analyze its effects on macrophage monolayers. The factor will be further compared to other factors known to act on the macrophage, particularly MIF, macrophage chemotactic factor, and colony stimulating factor. The action of this factor to induce macrophage activation in terms of enhanced microbicidal and tumoricidal activity is being examined. Peritoneal exudate macrophage subpopulations fractionated on discontinuous albumin gradients differ in the proliferative and activation responses to stimulation with MMF. Prostaglandin synthesis by macrophages appears to play a role in the regulation of MMF-induced proliferation. Based upon extensive experience with the lymphocyte and preliminary experiments with the macrophage, we plan to examine the mechanisms of action of this factor in relation to cyclic GMP generation and calcium influx, two key components of mitogen action. The responses of macrophages to stimulation by immunopotentiators, particularly those thought to act on cyclic nucleotide pathways, will be examined. The proposed experiments will contribute to the understanding of macrophage regulation and ultimately to immunotherapeutic endeavors to modulate the functions of this cell in enhancing resistance in the immunodeficient and/or cancer bearing patient.